The invention relates to a device for cooling, granulating and drying molten strands of synthetic thermoplastic material emerging from dies with a cooling unit which feeds strands into a bath of cooling liquid and to which is connected a granulator housing, which contains a cutting roll and to which is connected an outlet duct with a sieve to separate the thermoplastic material from the cooling liquid.
Such a device is known from the DE-PS 26 27 263. In this device the cooling unit comprises a discharge channel, whose upper end is supplied with cooling water and whose bottom end opens into a granulator housing containing a cutting roll. An outlet duct is attached to the granulator housing, and in particular in the horizontal direction, since the granulator housing deflects, owing to a rounded rear wall, the granules which are cut from the strands by the cutting roll. The granules leave the housing essentially in horizontal direction, a feature that is facilitated by a cooling water feeder provided in the housing close to the cutting point of the cutting roll. The cooling water floods the granules that have been cut off along the rounded rear wall of the granulator housing in the horizontal direction beyond a sieve arranged in the bottom of the horizontal outlet duct, where the water accompanying the granulate is drawn off in part by the sieve as a consequence of gravity. The intensity of this water separation is relatively low, since, as stated, only gravity acting on the water is utilized for this purpose. The result is that the granulate leaving the outlet duct still exhibits a high degree of surface moisture.
The invention is based on the problem of increasing the degree of drying of the granulate which has been separated by means of a cutting roll from wet or moist strands, and at the same time providing a special drier that has a low energy requirement.
This problem is solved by The invention by arranging the sieve in such a manner that the granules separated from the strands by the cutting roll impinge directly on the sieve at essentially the same speed as that imparted to the granules by the cutting roll.
The invention exploits the significant amount of kinetic energy that is transferred from the high speed cutting roll to the granules, in order to spin the water from the granules. The granules are namely cut by the individual knives of the cutting roll and thrown approximately tangentially from the cutting roll. The results for the granules are speeds in the order of magnitude of 10 m per second. If a granule impinges at such a speed on a sieve, water clinging to the granule is thrown almost completely from it, whereby the water passes through the sieve and thus can be extracted. To achieve this goal, the sieve is arranged in such a manner that the granules separated from the strands by the cutting roll impinge directly on the sieve without any intermediate deflection or point of impact.
The kinetic energy imparted to the granules is automatically generated owing to the necessary drive to the cutting roll, so that to accelerate the granules to virtually the circumferential speed of the cutting roll no additional energy is required.
Expediently the sieve is arranged at such an angle with respect to the direction of impingement of the granules that said granules impinge in the direction of a sieve that follows. In this case another sieve is utilized to throw the residual water from the granulate, since in this case the granules impinge on a sieve twice in succession. This serial connection of the sieves can be provided also multiple times.
It is also possible to design at least one of the sieves as a curved sieve, as shown, e.g. in the DE-PS 31 05 609. This curved sieve can be arranged in such a manner that granules either impinge obliquely on it or are fed tangentially to the curved sieve.
To extract the cooling liquid which may or may not cling to the sieves, a feeder for air to be blown on the sieve is assigned to at least one sieve. The air blown thus into the flow of the granulate escapes then over the sieves and drags with it any water clinging to said sieve.
The amount of water to be extracted through the sieves following the granulator can be decreased by attaching a pre-separator in front of the granulator housing. Such a pre-separator is known from the DE-PS 33 36 032. Then only a relatively small amount of residual water reaches the granulator, so that only said water has to be extracted from the sieves.
The heat intrinsic to the granules can be utilized by a well-known method to extract through evaporation the residual water from the granules. To achieve this goal, the cooling of the strands is set in such a manner that the heat intrinsic to the granules is left; and, following impingement of the granules on the sieve or sieves, said heat suffices to almost totally evaporate the water still wetting the granules.